From the current MOT testers manual......
6.3 Registration Plates and Vehicle Identification Number 6.3 - page 1
The Motor Bicycle and Side Car M.O.T. Inspection Manual Issue Date: May 2017
Information Method of Inspection Reason for Rejection Registration Plates
This inspection only applies to rear registration plates
Unregistered motor bicycles Unregistered motor bicycles need not be fitted with a registration plate.
Foreign, diplomatic and military vehicles This inspection does not apply to foreign registered, diplomatic or military vehicles.
Motor bicycles manufactured before 1 January 1977 may have registration plates displaying white, grey or silver characters on a black background.